Examqo › Examqo library › Nuclear medicine › Thyroid, parathyroid and adrenal
Radioiodine in differentiated thyroid cancer
After total thyroidectomy, I-131 serves three risk-based goals — destroying the normal thyroid remnant (ablation), adjuvant treatment of suspected microscopic disease, and treatment of metastatic disease — and the condition for all three is a high TSH and a low-iodine diet.
Why it matters in the exam
The exam asks when I-131 is not needed (a papillary tumour smaller than 1 cm at low risk), how the patient is prepared, and what phenomena such as stunning mean. Preparation: TSH above 30 mIU/L, reached by stopping levothyroxine for about 4 weeks or by injecting recombinant TSH (rhTSH) on the two days before; a low-iodine diet for 1–2 weeks; avoiding iodinated contrast for about 6–8 weeks; and a pregnancy test. rhTSH is accepted for ablation and for patients who would not tolerate hypothyroidism.
Usual activities: ablation at low-to-intermediate risk about 1.1 GBq, adjuvant treatment up to about 3.7 GBq, and metastatic disease 3.7–7.4 GBq. A whole-body scan about 3–7 days after treatment is more sensitive than a diagnostic scan and sometimes reveals new foci. Stunning is reduced uptake of the therapeutic activity after a high diagnostic I-131 activity; to avoid it, I-123 or a low diagnostic activity is used, or pre-treatment scanning is skipped. Side effects: sialadenitis and dry mouth, taste disturbance, epiphora from nasolacrimal duct obstruction, and at high cumulative activities marrow suppression and a small risk of a second malignancy.
Follow-up rests on thyroglobulin (Tg) and Tg antibodies and on neck ultrasound. A rising Tg with a negative iodine scan defines radioiodine-refractory disease and points to FDG PET/CT, which is also prognostic. In medullary cancer the cells do not take up iodine, so I-131 is not a treatment; the markers are calcitonin and CEA, and systemic imaging such as PET/CT (F-DOPA or FDG) is recommended when postoperative calcitonin rises above 150 pg/mL.
Key points
- I-131 in differentiated cancer: ablation, adjuvant or metastatic treatment by risk.
- Preparation: TSH above 30 mIU/L, low-iodine diet, no contrast, pregnancy test.
- A post-therapy scan is more sensitive than a diagnostic scan.
- Rising Tg with a negative scan — radioiodine-refractory and FDG PET/CT.
- Medullary cancer — calcitonin and CEA, not I-131.
Summary table
| Situation | I-131 | Usual activity | Note |
|---|---|---|---|
| Papillary under 1 cm at low risk | Not needed | — | Tg and ultrasound follow-up |
| Low-to-intermediate risk after total thyroidectomy | Ablation to be considered | About 1.1 GBq | rhTSH accepted |
| High risk or residual disease | Adjuvant treatment | Up to about 3.7 GBq | Post-therapy scan |
| Iodine-avid metastases | Treatment | 3.7–7.4 GBq | Repeat every 6–12 months by response |
| Rising Tg with a negative iodine scan | Not effective | — | FDG PET/CT; radioiodine-refractory |
In the full version for subscribers
- A simple algorithm
- Memory aid
- Exam traps
- 5 review cards
- Enough for IMLE
The full version is open to subscribers of: Nuclear medicine, Oncology, Internal medicine.
Subscriber? Read the full topic Plans and prices 25 free questions
Sources
- American Thyroid Association Management Guidelines for Adult Patients with Thyroid Nodules and Differentiated Thyroid Cancer, 2015 — Radioactive iodine remnant ablation, adjuvant therapy and follow-up
- EANM Guideline for Radioiodine Therapy of Differentiated Thyroid Cancer, 2022 — Preparation, activity and post-therapy imaging
- American Thyroid Association Revised Guidelines for the Management of Medullary Thyroid Carcinoma, 2015 — Postoperative calcitonin and imaging
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.